Self-assembled monolayers of reduced graphene oxide for robust 3D-printed supercapacitors

Herein, additive manufacturing, which is extremely promising in different sectors, has been adopted in the electrical energy storage field to fabricate efficient materials for supercapacitor applications. In particular, Al2O3-, steel-, and Cu-based microparticles have been used for the realization of 3D self-assembling materials covered with reduced graphene oxide to be processed through additive manufacturing. Functionalization of the particles with amino groups and a subsequent "self-assembly" step with graphene oxide, which was contextually partially reduced to rGO, was carried out. To further improve the electrical conductivity and AM processability, the composites were coated with a polyaniline-dodecylbenzene sulfonic acid complex and further blended with PLA. Afterward, they were extruded in the form of filaments, printed through the fused deposition modeling technique, and assembled into symmetrical solid-state devices. Electrochemical tests showed a maximum mass capacitance of 163 F/g, a maximum energy density of 15 Wh/Kg at 10 A/g, as well as good durability (85% capacitance retention within 5000 cycles) proving the effectiveness of the preparation and the efficiency of the as-manufactured composites.


Materials preparation
Al 2 O 3 and steel microparticles were obtained via thermal plasma synthesis from commercial Al 2 O 3 powder (Sigma-Aldrich) and commercial steel powder (Sigma-Aldrich).In particular, the samples were produced in a pre-pilot plant, which was designed and installed at the ENEA-Portici Research Centre and built by Praxair surface technologies 44 .A scheme of the plant, based on thermal plasma technology, is reported in Fig. 1.The plasma system is equipped with a DC non-transferred torch with a maximum power of 40 kW, a power supply of 80 kW, a dry scroll vacuum pump, as well as a bag filter.The torch is located on the top of a cold water-cooled jacketed-cylindrical stainless-steel reactor and the system works under a light vacuum (60-20 mbar).Processed powders are collected in a tank located below the reactor.During reactions, the above-mentioned powders, continuously fed to the reactor using a pneumatic feeding system, are sprayed through a nozzle located at the top of the reactor, horizontally with respect to the plasma flame.Tests were carried out adopting argon (Ar) as the main gas to light up the plasma, whereas helium (He) was selected as a secondary gas to enhance the flame conditions.Within the reactor, the powders underwent an evaporation/reconditioning reaction in a few milliseconds of residence time and then were dragged out by the process gas.The rapid cooling beyond the reaction zone limited the growth of the particles.As for Cu particles, they were commercial particles purchased from Sigma-Aldrich.

Coating and functionalization of particles
In view of the production through additive manufacturing of the supercapacitor devices, the above-described particles underwent a functionalization process which can be summarized in different steps.

First and second steps: functionalization of particles and their coating with rGO
The particle surface was first functionalized with amino groups and then linked to graphene oxide, which covered the functionalized particles in the form of reduced graphene oxide.
In detail, rGO-coated particles were prepared by means of a two-step process, as represented in Fig. 2, whereas photos of the process related to Cu particles are reported in Fig. 3.
The first step involved functionalization of the particles with (3-aminopropyl) triethoxysilane (APTES).In detail, 200 mg of particles were dispersed in a solution containing 10 mL of ethanol and 5 mL of deionized water.Afterward, the solution was sonicated for a few minutes before adding 0.5 mL of APTES to it.A functionalization step was then carried out at 80 °C for 12 h under magnetic stirring; after this procedure, the products were centrifuged several times in ethanol to obtain particles functionalized with amino groups.This functionalization made the particles positively charged 45 .
The second step involved a self-assembly process between GO and the amino-functionalized particles.In particular, 10 mg of GO were added to 20 mL of water and the pH of the solution was turned into the range 4-6 through the addition of diluted sodium hydroxide.At the same time, a second mixture was prepared by adding 200 mg of amino-functionalized particles to 20 mL of deionized water, and the pH of this suspension was adjusted to 3 by adding a diluted hydrochloric acid solution.Eventually, under magnetic stirring, the GOcontaining solution was added dropwise to the amino-functionalized particles-containing suspension.A 1-h sonication step of the as-obtained solution followed.During this phase, the negatively charged graphene oxide, after the ionization of its functional groups (such as carboxylic groups) has occurred in solution, underwent self-assembly with the particles owing to electrostatic interactions generated between the above-mentioned negative charges and the positively charged particles through their superficial amino groups; contextually, GO was partially reduced to rGO.

Third step: synthesis of the PANI-DBSA complex
The third step consisted of the synthesis of the polyaniline-dodecylbenzene sulfonic acid (PANI-DBSA) complex (see Fig. 4).PANI is a polymer characterized by high electrical conductivity, being therefore adopted to produce several devices such as supercapacitors, photoelectric devices, sensors, etc.Unfortunately, conductive polymers are not compatible with additive manufacturing 46 due to their poor processability and their low thermal stability.In fact, at high temperatures, PANI decomposes, hence losing its conductive nature.For this reason, PANI, before being anchored on the functionalized particles, was functionalized with the DBSA molecule to ensure high conductivity at the processing temperatures necessary for 3D printing.In particular, PANI (emeraldine base) was mixed with DBSA at 140 °C for 5 min with a PANI/DBSA weight ratio equal to 1:3, and the as-prepared solution was subsequently dried at 50 °C for 4 h.www.nature.com/scientificreports/Fourth step: functionalization of the rGO-coated particles with the PANI-DBSA complex The fourth step consisted of a self-assembly process between the functionalized particles coated with rGO and the solution containing the PANI-DBSA complex, as summarized in Fig. 5. Once the PANI-DBSA complex was obtained, it was dispersed into N-methyl-2-pyrrolidone to obtain a 30 mg/mL solution.The as-obtained solution was then added to 6 mL of a 16 mg/mL dispersion of the rGO-coated particles.The mixture was then stirred for 10 min at room temperature to obtain a composite consisting of the rGO-coated particles functionalized with PANI-DBSA.The three composites obtained will be named as follows: Steel-rGO@PANI-DBSA, Al 2 O 3 -rGO@ PANI-DBSA, and Cu-rGO@PANI-DBSA.

Blending with PLA, 3D printing and preparation of devices
In recent decades, the growing demand for polymeric materials capable of ensuring new and fascinating properties as well as providing high performance has pushed research forward into the creation of blends obtained by mixing different polymers.The main reason for this type of development lies in the possibility of obtaining a final product whose properties can be appropriately tuned depending on the type of components and how they are mixed.This allows us to respond more quickly to market needs, significantly reducing the time and investment required for the identification of new innovative polymers.Furthermore, blending also offers advantages in terms of improved processability and product homogeneity.For the above-mentioned reasons, the blending process was taken into consideration to improve the processability of PANI at high temperatures 47 .In particular, the amino-functionalized particles covered with rGO, and further functionalized with the PANI-DBSA complex were mixed with a thermoplastic polymer, polylactic acid (PLA).PLA is the most widely used biodegradable thermoplastic aliphatic polyester, as it is readily available, biocompatible and shows a rather good mechanical strength.
Owing to its low cost, its low melting temperature and its minimal warping, PLA is one of the easiest materials to be successfully adopted in AM, especially in FDM [48][49][50] .Therefore, Steel-rGO@PANI-DBSA, Al 2 O 3 -rGO@ PANI-DBSA and Cu-rGO@PANI-DBSA were mixed with commercial PLA (Sigma Aldrich) in order to obtain a much more AM processable material.In this procedure, composites were blended with PLA at a weight ratio of 1:2 (composites to PLA), which was the optimized ratio to ensure the best 3D printing processability.The mixture was heated to 200 °C and thoroughly mixed until a uniform compound was achieved.Subsequently, the compound was cooled down to room temperature.The resulting PLA-loaded composites were then placed within a MiniCTW twin-screw extruder (Ther-moScientifc) at a temperature of 200 °C and a screw speed of 30 rpm, obtaining a homogenous filament with a diameter of 1.75 mm.The 3D-printed model was designed via the CAD software Solidworks, and the corresponding gcode files were obtained through PrusaSlicer and eventually printed through a FDM technique using a Prusa i3 MK3S + 3D-printer (Prusa) to create a circular disc electrode with a diameter of 1 cm and a thickness of 2 mm.After printing the conductive composites in the form of disks, devices were built with the purpose of creating symmetrical supercapacitors.In particular, each device is composed of two 3D-printed discs, with a solid electrolyte inserted between the two discs to create a sandwich-like compact structure, as schematized in Fig. 6.The electrolyte was prepared by mixing 6 g of polyvinyl acetate (PVA) with 10 ml of a 1 M H 2 SO 4 solution.The as-prepared symmetrical supercapacitors were named as steel-rGO@PANI-DBSA-PLA, Al 2 O 3 -rGO @PANI-DBSA-PLA and Cu-rGO@PANI-DBSA-PLA.

Characterization techniques
Scanning electron microscopy (SEM) images were acquired through a LEO 1525 electron microscope (TESCAN), equipped with an energy-dispersive X-ray (EDX) probe.Powder X-ray diffraction (XRD) patterns were obtained with a Bruker D8 X-ray diffractometer using CuKα radiation.Thermogravimetry and derivative thermogravimetry (TG-DTG) were performed through an SDTQ 600 Analyzer (TA Instruments) with a 10 °C/min heating rate under air flow from room temperature to 800-900 °C.
Electrochemical measurements on composites before being mixed with PLA were obtained in a 0.5 M H 2 SO 4 electrolytic solution.Before measurements, 8 mg of the synthesized samples were dispersed into 160 μl of a 5 wt% Nafion solution, 900 μl of 2-propanol, and 100 μL of water to obtain a homogeneous ink which, after a  www.nature.com/scientificreports/30-min sonication and subsequent air-drying, was partially deposited dropwise onto a DRP-110 Screen Printed Electrode (SPE) made up of a carbon working electrode, a platinum counter electrode, and a silver reference electrode.SPEs were chosen due to their better properties over common carbon electrodes 51 .Electrochemical measurements on both composites without PLA and printed devices were carried out using an Autolab PGSTAT302N potentiostat (Metrohm, Herisau, Switzerland).
The mass capacitance (Csp) of the devices has been obtained from galvanostatic charge-discharge curves according to the following formula 52,53 : where i is the GCD current, Δt is the discharging time, ΔV is the potential window, and m is the catalyst loading mass.
Furthermore, the energy density (E) and the power density (P ) of each device have been evaluated according to the following equations: where Csp is the mass capacitance calculated in (1), Δt is the discharging time and ΔV is the potential window.
Furthermore, SEM analysis allowed us to better understand the morphology of the sample which, as can be seen from Fig. 7b, is made up of rounded particles with average sizes in the range 5-150 µm.The measured BET surface area of the particles is 9.8 m 2 /g, the total pore volume is 0.014 cm 3 /g with a micropore volume of 0.002 cm 3 /g.
The FT-IR of the α-Al 2 O 3 sample is shown in Fig. 7c: the IR profile shows a weak 3297 cm −1 band, characteristic of the stretching vibration of the -OH group linked to Al 3+54 .A vibrational band is also visible at 1574 cm −1 , corresponding to the physisorbed water.At 1408 cm −1 the characteristic band of water deformation vibrations can be detected 55 .Finally, between 1000 cm −1 and 500 cm −1 , bands due to the vibrational frequencies of the O-Al-O bonds are present 56,57 .
Furthermore, the thermogravimetric analysis of the alumina particles is shown in Fig. 7d.The thermogravimetric profile obtained under airflow shows a slight weight loss (as can be seen from the magnification shown in Fig. 7d in the green box) and the absence of significant decreasing steps, suggesting the presence of a single crystal phase and the absence of impurities.Weight loss (~ 1%) occurs at temperatures below 400 °C and is due to the evaporation of volatile components, such as water residues, including adsorbed water, free water, and crystalline water.

Characterization of steel particles
Figure 8a shows the X-ray powder pattern of the steel particles obtained through thermal plasma synthesis in the range of the 2θ angle between 20° and 80°.In the graph, the main peak related to the martensitic phase can be easily recognized at 42.86°5 8 , whereas the peaks related to the crystal planes of the austenitic phase can be identified at 44.2°, 50.38° and 64.4°5 9 .
Figure 8b shows the SEM images at different scale bars ranging from 500 μm to 50 μm.These images show particles with a quasi-spherical morphology and a size distribution in the range of 2-40 μm.The powder exhibits a BET surface area of 10.6 m 2 /g, total pore volume 0.021 cm 3 /g.Figure 8c and d show, respectively, the FT-IR spectrum and the TG-DTG graphs of the steel particles.The FT-IR spectrum shows no bands attributable to organic components and there are no significant weight losses in the thermogram, which confirms the purity of the steel sample.

Characterization of Cu particles
The copper particles were initially characterized through XRD analysis.The XRD diffraction pattern of the sample is shown in Fig. 9a: in particular, three distinct peaks can be identified at 2θ angle values equal to 42.81°, 49.98°, and 73.63°, corresponding to the (111), ( 200) and (220) crystal planes of metallic Cu 60 .From the analysis of the spectrum, it can be stated that the sample has good crystallinity due to the sharpness of its major peaks.
Furthermore, the Cu powder was characterized through scanning electron microscopy.The SEM images of the sample (reported in Fig. 9b) show particles with a spherical morphology with sizes from 1 to 30 μm, the most centered at 8 μm.The powder BET surface area is 18,4 m 2 /g, total pore volume is 0.062 cm 3 /g.
(1) www.nature.com/scientificreports/Moreover, the analysis of the copper particles through FT-IR shows an absorption band at 620 cm −1 , due to the vibration of the Cu-O bond of CuO 61 (Fig. 9c) and an absorption band at approximately 1056 cm −1 , which can be attributed to the C-OH stretching and to the OH bending vibration.
A thermogravimetric analysis under airflow carried out from room temperature to 1000 °C was also performed (Fig. 9d).The TG curve recorded an increase in the sample weight starting from the temperature of around 320 °C, attributed to the oxidation of the material which undergoes a percentage increase of about 25 wt% till 1000 °C.
The analysis of the rGO-coated particles evidences the occurrence of graphene oxide reduction (Figure S1, S2), probably due to its anchoring to the active sites of the amino-functionalized particles.Thermogravimetric analysis, shown in Figures S3, S4 and S5, shows weight losses in the temperature range of 200-700 °C.These losses can be attributed to the degradation of oxygenated functional groups and the degradation of C-C bonds in rGO.The weight losses vary among the three samples, with approximately 30%, 49%, and 70% for Al 2 O3, steel, and Cu particles, respectively.These discrepancies are likely due to differences in the particle size distribution, where Cu particles exhibit a more homogeneous and smaller size distribution, while Al 2 O 3 particles have less circular larger particles.

Characterization of PANI-DBSA complex
Figure 10a shows an SEM image of the PANI-DBSA complex, while Fig. 10b reports an SEM image of the same sample at a higher magnification with the corresponding four EDX maps reported in Fig. 10c.The EDX maps highlight the presence of the C, N, O and S elements, thereby successfully confirming the functionalization of the  Figure 12 shows also the FT-IR spectrum of the steel-rGO@PANI-DBSA composite in which several vibrational bands can be observed: at 3418 cm −1 , a vibrational band due to the stretching of the O-H bond of rGO; at around 2900 cm −1 and 2800 cm −1 , two intense vibrational bands due to the stretching of the CH 2 and CH 3 groups, respectively; at 1658 cm −1 , an intense band due to the vibration of the carbonic structure of graphene sheets; the bands at 1511 and 1408 cm −1 are attributable, respectively, to the C = N and C = C stretching of the quinonoid and benzenoid units of PANI; the bands at 1300 and 1269 cm −1 are related to the C-N stretching of the PANI ring 64 ; a weak vibrational band, at 1218 cm -1 , originates from the vibration of the C-O-C group; weaker vibrational bands are observed in the range 1000-1200 cm −1 , suggesting the presence of bonds such as C-H or C-O; a vibrational band at 668 cm −1 is attributable to the stretching of the S = O bond of the DBSA molecule.Overall, all these FT-IR observations clearly indicate the coating of steel particles with rGO and PANI-DBSA, in agreement with SEM/EDX analysis.The morphology of the composites Al 2 O 3 -rGO@PANI-DBSA-PLA, steel-rGO@PANI-DBSA-PLA and Cu-rGO@ PANI-DBSA-PLA was also explored by means of SEM analyses (Fig. 14a-f).SEM images of all the samples show a rough surface with cracks and ridges.FT-IR analysis was performed on the conductive composite Al 2 O 3 -rGO@PANI-DBSA-PLA (Fig. 14g), confirming the presence of PLA, with the vibrational bands of its CH, CH 2 and CH 3 bonds.In Fig. 14h, the FT-IR profile of steel-rGO@PANI-DBSA-PLA has also been reported.As can be seen, the bands of PLA (green profile in the same figure) are very intense and tend to mask the bands of the steel-rGO@PANI-DBSA sample.The bands in the range 2845-2990 cm −1 can be attributed to the stretching of the CH, CH 2 and CH 3 groups characteristic of PLA, PANI-DBSA, rGO and APTES as well, whereas the vibrational band characteristic of graphene sheets, at 1655 cm −1 , is hidden by the band at 1750 cm −1 relative to the C = O stretching of PLA.Furthermore, at 1187 cm −1 , a vibrational band due to the C-O-C stretching of PLA can also be detected.Moreover, a slight shift of the PANI band from 1466 cm −1 to 1447 cm −1 can be observed, probably due to interaction with PLA.The same phenomenon can be observed by looking at the band at 1216 cm −1 , characteristic of the vibration of the C-O-C group of the rGO, which has moved to 1213 cm −1 .Moreover, the vibrational band at 746 cm −1 related to the stretching of the S = O bond of the DBSA molecule is also slightly visible.Finally, the bands between 1000-500 cm −1 are due to the vibrations of the -OH, C-C, C-COO and C = bonds of the PLA 65 .The most evident bands are visible at 1750 cm −1 and at 1599 cm −1 , due, respectively, to the stretching of the C = O bond of PLA and to the vibrations of the C = N bond of PANI.Eventually, Fig. 14i shows Cu-rGO@PANI-DBSA-PLA, which successfully confirms the occurred blending with PLA as well.

Electrochemical characterization of devices for supercapacitor applications
CV measurements recorded on the composites before being mixed with PLA were reported in Figures S6, S7 and S8, along with the profiles of the pristine microparticles.An increased charge accumulation compared to microparticles alone, likely due to graphene, was observed.
Moreover, to evaluate the capacitance performance of the final devices, GCD measurements were performed, with the PVA-H 2 SO 4 acting as a solid-state electrolytic layer.Figure 18 shows the GCD curves, obtained at 1 A/g of Al 2 O 3 -rGO@PANI-DBSA-PLA (pink curve), steel-rGO@PANI-DBSA-PLA (red curve) and Cu-rGO@ PANI-DBSA-PLA (blue curve).The three samples exhibit a stable performance, as can be seen from the similar size of the iR drop for each charge/discharge curve, and short discharge time.From them, specific capacitance values were calculated, according to Eq. (1): 173, 134 and 160 F/g for, Al 2 O 3 -rGO@PANI-DBSA-PLA, steel-rGO@ PANI-DBSA-PLA and Cu-rGO@PANI-DBSA-PLA respectively.The capacitance values, in any case, higher than 100 F/g, are of similar magnitudes among the three samples, indicating the effectiveness of the approach.On the other hand, the observed differences can be attributed to the different particle size distributions, which, in turn, affect the amount of reduced graphene oxide assembled and the active surface areas available for electrochemistry.Additionally, the effective ion wettability of the samples, which depends on accessibility between particles, favored in the case of larger particles (see the small capacitance reduction for the Al 2 O 3 -support-based capacitors in Fig. 19) plays a crucial role.This effect is dominant even over the electrical conductivity of the materials, contributing the most to these differences.The above-mentioned Fig. 19 reports the mass capacitance values at different current densities for the three samples.The specific capacitance, as expected, decreased at increasing current densities in the range of 0.5-10 A/g, reaching 163, 119 and 140 F/g at 10 A/g for, Al 2 O 3 -rGO@PANI-DBSA-PLA, steel-rGO@PANI-DBSA-PLA and Cu-rGO@PANI-DBSA-PLA, respectively.This is an expected behavior since higher current densities correspond to shorter intervals for electrolyte ions to diffuse into the electrode channels and, therefore, they can access a smaller portion of the active material's surface area.Conversely, as previously observed, the electrical conductivity of the supports appears less relevant.Indeed, copper which is the most conductive and enjoys the higher rGO content and surface area, exhibits a capacitance of the same order of magnitude.
Starting from the specific capacitance values, energy densities and power densities of the devices were also evaluated according to (2) and (3) (see Ragone plots in Fig. 20).Values of energy and power densities were recorded, respectively, in the range of 10.5-15 Wh/kg and 2.21-2.58W/kg.Furthermore, the as-obtained supercapacitors were tested after several cycles of usage, showing encouraging results in terms of durability.In particular, as reported in Fig. 21, Al 2 O 3 -rGO@PANI-DBSA-PLA, steel-rGO@ PANI-DBSA-PLA and Cu-rGO@PANI-DBSA-PLA retain 85%, 77%, and 81% of their initial capacitance values after 5000 cycles, respectively.
These values, along with the specific capacitances and the energy densities, prove that the obtained results are comparable with those reached with non-3D-printed composites including rGO and other graphene-based materials, even when comparing the ones with finer sizes (see Table 1).The as-obtained results are likely due to the enhanced stability of the GO self-assembling 3D structures on surface-functionalized robust materials, as well as the combination in the composite structure of rGO and PANI.The former guarantees good wettability with the electrolyte prevents aggregations of adjacent graphene sheets, and contributes to the capacitive behavior of the system due to its high surface area and conductive sheets.The latter guarantees enhanced electrical conductivity while also contributing to the overall capacitance 66 .Therefore, the obtained results suggested the successful manufacturing through AM of the above-prepared materials and highlighted the possibility of easily and economically creating energy storage devices made up of 3D printable materials with good performance.As for future perspectives, future improvements in performance can be achieved, for instance, by adjusting the percentage of electrochemically active material in the composites.

Conclusion
In this study, with the aim of investigating a versatile and adaptable approach for obtaining robust and performing 3D printed supercapacitors, various architectures, that benefit from graphene covering, were reported and studied.In particular, Al 2 O 3 -, steel-, and Cu-based microparticles have been explored for the realization of 3D self-assembling materials covered with rGO to be processed through AM.
In detail, the manufacturing procedure of the devices can be summarized as follows: (i) the surface of the particles of Cu, Al 2 O 3 and steel was first functionalized with amino groups and then covered through selfassembly with GO, which covered the particles in the form of reduced rGO; (ii) a PANI-DBSA complex was created; (iii) to further improve the conductivity and processability characteristics for 3D printing, a self-assembly process was carried out between the particles coated with carbonaceous material and the solution containing the PANI-DBSA complex; (IV) the as-obtained composites were mixed with an optimum amount of PLA in order to further improve AM processability of the composites; (V) the PLA containing-composites were extruded in filaments, which were then printed through FDM technique to create circular disc electrodes which were, eventually, assembled in solid-state electrolyte-based symmetric devices.After a broad characterization of both the pristine particles and the final PLA-containing composites through FT-IR, TG-DTG, SEM and EDX techniques, which allowed to confirm their nature, the devices were tested in terms of energy storage through GCD tests.
Capacitance values, in the order of hundreds of F/g, are of similar magnitudes among the three samples, indicating the effectiveness of the approach.On the other hand, the observed small differences can be attributed to the different particle size distributions, which, in turn, affect the amount of reduced graphene oxide assembled and the active surface areas available for electrochemical reactions.Additionally, the effective ion wettability of the samples, which depends on accessibility between particles in the pathway towards the active surface, favored in the case of larger particles, plays a crucial role in these differences.Conversely, the electrical conductivity of the supports seems less relevant.Indeed, copper which is the most conductive and benefits from the higher rGO content and surface area, exhibits capacitance of a similar order of magnitude.The energy density values and good durability demonstrate that the results obtained are comparable to those achieved with non-3D-printed, high-performance composites containing rGO and other graphene-based materials, even when considering materials with finer sizes.This is likely due to the advantage of the inclusion of high-performance rGO and PANI in self-assembling 3D structures based on surface-functionalized very robust materials.Therefore, the obtained results suggested the successful manufacturing through AM of the aboveprepared materials and highlighted the possibility of easily and economically creating energy storage devices made up of 3D printable materials with good performance.As for future perspectives, better performance can be reached, for instance, by modifying the percentage of electrochemically active material in the composites.These results they also open the way for the creation of more complex structures high surface area structures.

Figure 2 .
Figure 2. Scheme of the two-step process of coating functionalized particles with rGO.

Figure 3 .
Figure 3. Photos of the two-step process of coating for functionalized Cu particles with rGO.

Figure 4 .
Figure 4. Scheme of the synthesis of the PANI-DBSA complex.

Figure 5 .
Figure 5. Scheme of functionalization of the rGO-coated particles with the PANI-DBSA complex.

Figures 11 ,
Figures 11, 12 and 13 report the EDX analyses for Al 2 O 3 -rGO, steel-rGO, and Cu-rGO, all of them coated with PANI-DBSA.The EDX maps highlight the presence of the individual elements and the homogeneity of the samples, demonstrating the correct functionalization of the particles with PANI-DBSA.Figure12shows also the FT-IR spectrum of the steel-rGO@PANI-DBSA composite in which several vibrational bands can be observed: at 3418 cm −1 , a vibrational band due to the stretching of the O-H bond of rGO; at around 2900 cm −1 and 2800 cm −1 , two intense vibrational bands due to the stretching of the CH 2 and CH 3 groups, respectively; at 1658 cm −1 , an intense band due to the vibration of the carbonic structure of graphene sheets; the bands at 1511 and 1408 cm −1 are attributable, respectively, to the C = N and C = C stretching of the quinonoid and benzenoid units of PANI; the bands at 1300 and 1269 cm −1 are related to the C-N stretching of the PANI ring64 ; a weak vibrational band, at 1218 cm -1 , originates from the vibration of the C-O-C group; weaker vibrational bands are observed in the range 1000-1200 cm −1 , suggesting the presence of bonds such as C-H or C-O; a vibrational band at 668 cm −1 is attributable to the stretching of the S = O bond of the DBSA molecule.Overall, all these FT-IR observations clearly indicate the coating of steel particles with rGO and PANI-DBSA, in agreement with SEM/EDX analysis.

Figure 10 .
Figure 10.SEM images with scale bars of (a) 100 and (b) 50 µm; and (c) corresponding EDX maps of the PANI-DBSA complex.(d) FT-IR spectra of PANI (green profile) and PANI-DBSA complex (red profile).